This invention relates to a sheet transport device for use in various image reading/forming apparatuses such as copiers, scanners, facsimiles, and printers for transporting sheets such as documents and copy sheets, to a specified image reading/forming position successively to read original document images and/or form an image on the copy sheets successively.
In the above image forming apparatus, such as copiers, a sheet inverting mechanism for turning the side of a document and a copy sheet (hereinafter, also simply referred to as "sheet" as a general term for document and copy sheet) is required in a document feeder/sheet transport device in order to read an image on the opposite sides of the document or form an image on opposite sides of the copy sheet.
Heretofore, it has been a general practice to adopt a switchback mechanism in the document feeder/sheet transport device wherein the sheet transport direction is inverted in such a manner that the lead end of the sheet in the sheet entering direction into a switchback route becomes the tail end thereof in the sheet exiting direction in order to turn the side of the sheet.
For instance, Japanese Unexamined Patent Publication No. HEI 7-64359 discloses an automatic document feeder for feeding a document to an image reading position for double side image reading. The device of this publication is operated such that: a document is fed from a document tray to the specified image reading position for one side image reading; temporarily discharged onto a document discharge tray after the one side image reading; transported in the opposite direction opposite to the document discharge direction onto the document discharge tray by a switchback mechanism; and then fed to the image reading position again to read an image on the opposite side of the document.
Similar to the above automatic document feeder, there has been known a sheet transport device for use in an image forming apparatus, such as copiers, that uses a switchback mechanism to transport a copy sheet to a specified image forming position for double side image formation. In such a sheet transport device provided with the switchback mechanism, similar to the document feeder, the sheet transport direction is inverted by the switchback mechanism after one side image formation to flip over the side of the sheet for the opposite side image formation.
In the above switchback mechanism, a transport operation (corresponding to a first half stage of switchback operation) of transporting the document in a certain direction along the switchback route and a transport operation (corresponding to a latter half stage of switchback operation) of transporting the document in the opposite direction to the first half stage along the main transport route to the image reading position are performed sequentially. Accordingly, in the case where a skew transport state occurs during the switchback operation, it is difficult to correct the skew transport state. It is highly likely that the document/sheet is transported to the image reading/forming position in the skew transport state in the above switchback mechanism, which may result in a poor image reading/formation in which the image of the document is read/formed on the sheet in the skew transport state.
Specifically, in the above automatic document feeder, the lead end of the document on the way of a switchback operation with respect to the entering direction is left free without the guide means with the result that a skew transport of the document is liable to occur. Thereby, it is highly likely that the document (copy sheet) is transported to the image reading (forming) position in a skew transport state, resulting in a poor image reading (formation) such that an image is read (formed) in the skew transport state.